Physics-based modelling of fission gas swelling and release in UO2 applied to integral fuel rod analysis

A physics-based model is developed for analysing the coupled phenomena of fission gas swelling and release in UO2 fuel during irradiation. The model is featured by a level of complexity suitable for application to integral fuel rod analysis and consistent with the uncertainties pertaining to some parameters. The emphasis is on the modelling of the grain-face gas bubble development and the related dependence of the fission gas swelling and release on the local hydrostatic stress, which is of special importance for the analysis of the fuel behaviour during power ramps and pellet-cladding mechanical interaction conditions. The applicability of the new model to integral fuel rod analysis is verified through implementation and testing in the TRANSURANUS fuel rod analysis code. In the frame of the IAEA co-ordinated research project on Fuel Modelling at Extended Burn-up FUMEX-III, the model is applied to the simulation of irradiation experiments from the OECD/NEA International Fuel Performance Experiments database. The comparison of the results with the available experimental data of fission gas swelling and release at moderate burn-up is presented as a first step of validation, pointing out an encouraging predictive accuracy for different irradiation conditions, without any fitting applied to the model parameters.